An interdisciplinary effort is proposed to study the interaction between inhaled aerosols and the respiratory system. Project 1 seeks to determine the influence of the hygroscopic growth of aerosols on deposition; whether supersaturation of aerosols occurs and its implications for deposition; and the influence of altered respiratory mechanics on deposition. Project 2 examines the importance of the pH and molecular composition of sulfate/sulfite aerosols in causing pulmonary functional changes in guinea pigs; and whether the NH3 excreted into alveoli modifies the chemistry and effect of acid aerosols. Project 3 extends the observations of Project 2 to healthy subjects and to adolescent asthmatic patients with exposures during rest and exercise. Project 4 examines the role of the alveolar wall in determining whether insoluble particles that deposit locally are cleared along the muco-ciliary system or enter the tissue compartment.